Differences that I've come up with:

drum/drum:
1. smaller fluid reservoirs
2. residual pressure valves for both lines

disc/drum:
1. larger disc reservoir
2. no residual pressure valve disc

disc/disc:
1. larger reservoirs both circuits
2. no residual pressure valves at all.


With this in mind, is there any reason you can't keep your drum/drum master cylinder as you upgrade your brakes to disc/drum then to disc/disc if you:

1. remove residual pressure valve for disc circuit.
2. MAKE SURE YOU KEEP THE FLUID FULL IN THE TINY RESERVOIR(s).

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I did the disc swap without changing the mc. Now the pedal is soft (it was a rock with 9") and kinda spongy. The softness doesn't bleed out. You might want to change mc just for hydraulic proportioning reasons.

I have used a later rebuilt drum MC w/no resid valves and it works fine. Just keep em topped off. That said, if you have the option, why not use a later disc MC?

Lou

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Home of Slant6-powered fun machines since 1988

I use a 66 Corvette manual master cylinder for 4 wheel disc in several applications here. Available from NAPA for $65.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Sandy, what're the specs on that '66 Corvette master cylinder? Single/double reservoir? Bore diameter? Other info?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

its a double cylinder and it looks like a truck MC without the residual valves...otherwise,,,,um,,,,no idea.

I ordered one to try once and liked it so use it in several places (51 DeSoto 4 wheel disc)(mudbogger split system)

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Yeah....Im the one who destroyed this rare, vintage automobile.....

My first order of business is getting a dual master cylinder from the single I have. As it's drum/drum, I wanted a master with the residual valves. Then if I go to discs, I didn't want to get another master cylinder.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

You don't need residual valves for drums that have been rebuilt (wheel cyls) in the last 20 years - they have springs between the cups. In fact, I would bet that drum MCs won't have RVs anyway. Just get a disc MC and be done with it, for drums or discs.

Lou

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Home of Slant6-powered fun machines since 1988

Are the residual valves located on the outside of theMaster cylinder itself? Or are they built into it? We got a new M/C a little while ago for my sons Dart (drum/drum)and I didn't see anything on the outside. I just upgraded to Discs and would like to use this M/C in we can. I did put an adjustable proportioning valve in the rear line.

Thanks,

Rick

If you look into each of the two brake pipe ports on the side of the master cylinder, in each one you'll see a little cone of metal with a hole in the middle. That's the brake pipe seat. The RP valve is behind the brake pipe seat. To service the RP valve, you thread a short sheetmetal screw into the hole in the middle of the seat, picking the screw size carefully so as not to distort the seat, then pry the screw out, which will bring the seat with it. The RP valve, a rubber flap with a coil spring behind it, can then be accessed.
Read up the thread; drum/drum master cylinders aren't the same as disc/drum units.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.